1. Field of the Invention
The present invention relates to an Asynchronous Transfer Mode (ATM) switch, and more particularly to a method of controlling a bi-directional switching operation for a STM-1 (synchronous transfer mode-1) signal transfer line of an ATM switch.
2. Background of the Related Art
As well known to those skilled in the art, an ATM switch must be able to perform a protection switching operation using multiplex channels to overcome a fault at a STM-1 signal transfer line. The protection switching operation is performed through either a 1+1 or 1+N structure, and may be classified into an unidirectional mode and a bi-directional mode.
A STM-1 signal is transferred at 155.520 Mbps in a synchronous digital manner, and can be multiplexed from or demultiplexed into a plurality of E-1 signals.
In the 1+1 bi-directional switching operation, which is compatible with the 1+N bi-directional switching operation, a “K1” byte structure and “K2” byte structure are used in a Multiplex Section Overhead (MSOH) to perform a MSP switching operation. The MSP protocol is defined by ITU-T (International Telecommunication Union-Telecommunication Standardization Sector) Recommendation G.783, Annex A.
The “K1” and K2 byte structures are shown in FIG. 1. In the 1+N structure, channels “1” to “N” are working channels while a channel “0” is a protection channel. Similarly, in the 1+1 structure, a channel “1” is a working channel while a channel “0” is a protection channel.
A value “0” at a protection switching structure region of the K2 byte structure means a 1+1 structure, while “1” means a 1+N structure.
A value “111” at a switching status region of the K2 byte structure means an AIS (Alarm Indication signal) status and “110” means a RDI (Remote Detect Indication). A value “100” means the unidirectional mode and “101” means the bi-directional mode.
Further, a value “1111” at a request type region of the byte structures is defined as a “lockout”, “1110” is a “forced switch”, and “1101” is a “signal fail-high priority”. Moreover, a value of “1000” is a “manual switch”, “0010” is a “reverse request”, “0001” is a “do not revert”, and “0000” is a “no request”.
There are additional values assigned and recommended by ITU-T in addition to the above described values, but they are not commonly used. Among the above given values, those with higher values have higher priorities.
In the 1+1 or 1+N structure, the 1 working channel or N working channels, where N can be up to 14, share the 1 protection channel. No signal can be transferred through the protection channel while being transferred through any of the working channels in a normal state.
FIG. 2 is a functional block diagram of a switching operation device with the 1+1 structure of an ATM switch. Referring to FIG. 2, when a working side 11 of a source station 10 and a working side 21 of a target station 20, or a protection side 12 of the source station 10 and a protection side 22 of the target station 20 are working in an active state, if a command such as a forced switch, manual switch, or the like is entered by an operator, or if a signal fail (SF) at the working sides 11 and 21, or a signal degrade (SD) at the working sides 11 and 21 or protection sides 12 and 22 occurs, then a bi-directional protection switching operation is generally performed using the “K1” and “K2” byte structures. The “K1” and “K2” byte structures have channel values as shown in FIG. 3.
A related art method of controlling a bi-directional protection switching operation of an ATM switch will be described hereinbelow with reference to FIGS. 1 to 4.
First, it is determined whether a control signal, such as a forced switch or manual switch by an operator, has been entered when the working sides 11 and 21 or protection sides 12 and 22 are working active, or whether a new event such as a SF (not at the protection sides 12 and 22), SD, or fault at a transfer line has occurred.
If it is determined that a new event has occurred or that a control signal has been entered by the operator, then the source station 10 detects the new event and compares a priority of the new event with that of a current event having already occurred and still being under way.
By comparison of their priorities, if the new event is determined to have higher priority, then the source station 10 transmits a request signal Req+ATOB or Req+BTOA including a request type and channel information, to a target station 20 (ST1).
Upon receiving the request signal Req+ATOB or Req+BTOA from the first station 10, the second station 20 checks the validity of the request signal. Thus, the target station 20 compares the priority of the operator's new request or new event with that of the current event. It then transmits a reverse request (RR) signal RR+BWORK or RR+AWORK with updated channel values of the “K2” byte structure to the source station 10, if the priority of the new event is higher than that of the current event (Step ST2). The RR signal indicates that the request of the source station 10 has been accepted by the target station 20.
Upon receiving the RR signal RR+BWORK or RR+AWORK from the target station 20, the source station 10 performs a switching operation from the working side 11 to the protection side 12 thereof When the switching operation has been completed, the source station 10 transmits a switching notification signal Req+BWORK or Req+AWORK to notify the target station 20 of the completion of the switching operation (Step ST3).
Upon receiving the switching notification signal, the second station 20 is notified that the first station 10 has performed the switching operation, and thus performs a switching operation from the working side 21 to the protection side 22 thereof. When the switching operation has been completed, the second station 20 transmits a switching notification signal RR+BWORK or RR+AWORK to the first station 10 (Step ST4).
By way of example, when a SF is detected when the working side 11 and the source station 10 is active, the working side 11 transmits a request signal SF+ATOB to the target station 20, as show in step ST1. The working side 21 in the target station 20 compares a priority of a new request or new event with that of a current event, and then transmits a RR signal RR+BWORK to the source station 10 if the priority of the new event is higher than that of the current one as shown in step ST2.
Upon receiving the RR signal, the source station 10 performs a switching operation from the working side 11 thereof to the protection side 12 thereof, and transmits a switching notification signal SF+BWORK to the target station 20 as shown in step ST3. The target station 20 then performs a switching operation from the working side 21 to the protection side 22 thereof, and notifies a result of the switching operation to the source station 10 as shown in step ST4.
The detection of the SF signal indicates that a critical fault at a corresponding channel or line has occurred at the 1+1 structure operating in the bi-directional mode. If this happens when the protection side is active, then the switching operation must be immediately performed to make the working side active.
The related art system and method thus has various problems. For example, if the SF signal is detected when the working side is active, then the switching operation is performed based on a protection switching scenario. This results in an abnormal state lasting for a period of time, albeit a short period of time. If, however, the abnormal state becomes worse, then it can cause a call termination.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.